As the operation of certain class of semiconductor electronic and sensing devices is extended to temperatures greater than 500° C., the degradation of the contact metallization to these devices tend to increase at near exponential rate. The resultant outcome is the correspondingly gradual operational degradation of the devices, eventually leading to premature catastrophic failure. The primary mechanisms responsible for such failures are: (1) oxidation of the contact metallization by adventitious oxygen diffusion into the metallization from outside, which leads to increase in the resistance of the metallization; (2) the inter-mixing of the multiple metallization layers that constitute the metallization system leads to micro-structural phase transformations, void formation, Kirkendall vacancies, and grain boundary nucleation, which all cooperatively act to degrade the electrical functional characteristics of the metallization; and (3) intermetallic diffusion to the semiconductor interface, forming a new interfacial layer with the semiconductor, thus changing the metal/semiconductor interface electronic characteristics from either ohmic to Schottky or Schottky to ohmic contacts.
Conventional semiconductor pressure sensors are typically rated up to 125° C. One reason is because this class of semiconductors, such as silicon, is limited by their material properties at higher temperatures and enhanced surface reactivity with the contact metallization. Extending the device operation beyond its operating temperature limit leads to the degradation of performance and eventual catastrophic failure in a very short time. In the absence of alternative devices that can operate at higher temperatures, various cooling strategies have been deployed, which extends sensor operation to ˜350° C.
However, by applying this strategy new challenges are introduced. For example, water cooling can compromise signal integrity, since it couples externally sourced cold temperature and the turbulence generated by the flowing coolant with the temperature of the test environment to distort the actual reading. Alternatively, the pressure sensors are recessed a few inches to feet away to a lower temperature location via an infinite tube. The disadvantages of this strategy include the introduction of propagation delays of the pressure waves to the sensor. Considering that the infinite tube is essentially an acoustic filter, the attenuation of critical thermos-acoustic frequencies that are responsible for the instabilities could be missed, thus making it difficult to detect and mitigate such instabilities that might damage engine components. The bulkiness that is typically associated with the water- or gas-cooled sensors makes it difficult to be inserted deeper into the environment.
Recent advances made in uncooled Silicon Carbide (SiC) piezoresistive pressure sensor technology have led to its beta applications in combustor and jet engine ground experiments to directly measure dynamic pressure at temperatures up to 600° C. Therefore, the technology offers promise for eventual applications in other high temperature environments, such as jet engines, nuclear power plants, pharmaceutical plants, and automobiles, without the need for cooling.
As SiC pressure sensors are operated at 600° C., the long term operational reliability may be challenged by the same prevailing failure mechanisms that are manifested at higher temperatures. The degradation in the reliability is represented by the drifts in the zero pressure offset voltage (ZPO) when the SiC pressure sensor is operating at the fixed 600° C. over time. Because the ZPO voltage is the calibrated reference voltage prior to pressure sensor use, any deviation from such reference value during operation will introduce significant measurement errors during use. The random nature of the drift phenomena eliminates the possible implementation of commonly practiced temperature compensation as a solution.
It has been determined that the cause of the ZPO drift is largely due to the reaction kinetics and thermodynamic activities occurring within the electrical contact metallization and at the metal/SiC interface. At high temperature, inter metallic diffusion between the metal layers lead to material phase transformation. Also, at high temperature, there is a continuous reaction between the contact metallization and the underlying SiC semiconductor. The combination of these results in changes in the resistance of the metallurgical junction, leading to changes in the ZPO as function of time. The oxidation of the metallization by migrating oxygen from the outside leads to premature sensor degradation. These mechanisms must be eliminated or significantly controlled in order for the sensor to operate reliably in the duration of the test, with minimal change in the reference voltage.
Accordingly, improved DBSs may be beneficial.